Methods of determining scattering of radiation by structures of finite thicknesses on a patterning device

ABSTRACT

A method including: obtaining a thin-mask transmission function of a patterning device and a M3D model for a lithographic process, wherein the thin-mask transmission function is a continuous transmission mask (CTM) and the M3D model at least represents a portion of M3D attributable to multiple edges of structures on the patterning device; determining a M3D mask transmission function of the patterning device by using the thin-mask transmission function and the M3D model; and determining an aerial image produced by the patterning device and the lithographic process, by using the M3D mask transmission function.

This application is a continuation of U.S. patent application Ser. No.17/326,481, which was filed on May 21, 2021, now allowed, which is acontinuation of U.S. patent application Ser. No. 16/467,124, which wasfiled on Jun. 6, 2019, now U.S. Pat. No. 11,016,395, which is the U.S.national phase entry of PCT Patent Application No. PCT/EP2017/081744,which was filed Dec. 6, 2017, which claims the benefit of priority ofU.S. Provisional Application No. 62/439,682, which was filed on Dec. 28,2016, each of the foregoing applications is incorporated herein in itsentirety by reference.

TECHNICAL FIELD

The description herein relates generally to methods of determiningscattering of radiation due to finite thicknesses of patterns on apatterning device for lithographic processes and lithographic projectionapparatuses.

BACKGROUND

A lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, a patterningdevice (e.g., a mask) may contain or provide a device patterncorresponding to an individual layer of the IC (“design layout”), andthis pattern can be transferred onto a target portion (e.g. comprisingone or more dies) on a substrate (e.g., silicon wafer) that has beencoated with a layer of radiation-sensitive material (“resist”), bymethods such as irradiating the target portion through the pattern onthe patterning device. In general, a single substrate contains aplurality of adjacent target portions to which the pattern istransferred successively by the lithographic projection apparatus, onetarget portion at a time. In one type of lithographic projectionapparatuses, the pattern on the entire patterning device is transferredonto one target portion in one go; such an apparatus is commonlyreferred to as a stepper. In an alternative apparatus, commonly referredto as a step-and-scan apparatus, a projection beam scans over thepatterning device in a given reference direction (the “scanning”direction) while synchronously moving the substrate parallel oranti-parallel to this reference direction. Different portions of thepattern on the patterning device are transferred to one target portionprogressively. Since, in general, the lithographic projection apparatuswill have a reduction ratio M (e.g., 4), the speed F at which thesubstrate is moved will be 1/M times that at which the projection beamscans the patterning device. More information with regard tolithographic devices as described herein can be gleaned, for example,from U.S. Pat. No. 6,046,792, incorporated herein by reference.

Prior to transferring the device pattern from the patterning device tothe substrate, the substrate may undergo various procedures, such aspriming, resist coating and a soft bake. After exposure, the substratemay be subjected to other procedures, such as a post-exposure bake(PEB), development, a hard bake and measurement/inspection of thetransferred pattern. This array of procedures is used as a basis to makean individual layer of a device, e.g., an IC. The substrate may thenundergo various processes such as etching, ion-implantation (doping),metallization, oxidation, chemo-mechanical polishing, etc., all intendedto finish off the individual layer of the device. If several layers arerequired in the device, then the whole procedure, or a variant thereof,is repeated for each layer. Eventually, a device will be present in eachtarget portion on the substrate. These devices are then separated fromone another by a technique such as dicing or sawing, whence theindividual devices can be mounted on a carrier, connected to pins, etc.

As noted, lithography is a central step in the manufacturing of devicesuch as ICs, where patterns formed on substrates define functionalelements of the devices, such as microprocessors, memory chips etc.Similar lithographic techniques are also used in the formation of flatpanel displays, micro-electro mechanical systems (MEMS) and otherdevices.

As semiconductor manufacturing processes continue to advance, thedimensions of functional elements have continually been reduced whilethe amount of functional elements, such as transistors, per device hasbeen steadily increasing over decades, following a trend commonlyreferred to as “Moore's law”. At the current state of technology, layersof devices are manufactured using lithographic projection apparatusesthat project a design layout onto a substrate using illumination from adeep-ultraviolet illumination source, creating individual functionalelements having dimensions well below 100 nm, i.e. less than half thewavelength of the radiation from the illumination source (e.g., a 193 nmillumination source).

This process in which features with dimensions smaller than theclassical resolution limit of a lithographic projection apparatus areprinted, is commonly known as low-k₁ lithography, according to theresolution formula CD=k₁×λ/NA, where λ is the wavelength of radiationemployed (currently in most cases 248 nm or 193 nm), NA is the numericalaperture of projection optics in the lithographic projection apparatus,CD is the “critical dimension”—generally the smallest feature sizeprinted—and k₁ is an empirical resolution factor. In general, thesmaller k₁ the more difficult it becomes to reproduce a pattern on thesubstrate that resembles the shape and dimensions planned by a circuitdesigner in order to achieve particular electrical functionality andperformance. To overcome these difficulties, sophisticated fine-tuningsteps are applied to the lithographic projection apparatus, the designlayout, or the patterning device. These include, for example, but notlimited to, optimization of NA and optical coherence settings,customized illumination schemes, use of phase shifting patterningdevices, optical proximity correction (OPC) in the design layout, orother methods generally defined as “resolution enhancement techniques”(RET). The term “projection optics” as used herein should be broadlyinterpreted as encompassing various types of optical systems, includingrefractive optics, reflective optics, apertures and catadioptric optics,for example. The term “projection optics” may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, collectively orsingularly. The term “projection optics” may include any opticalcomponent in the lithographic projection apparatus, no matter where theoptical component is located on an optical path of the lithographicprojection apparatus. Projection optics may include optical componentsfor shaping, adjusting and/or projecting radiation from the sourcebefore the radiation passes the patterning device, and/or opticalcomponents for shaping, adjusting and/or projecting the radiation afterthe radiation passes the patterning device. The projection opticsgenerally exclude the source and the patterning device.

BRIEF SUMMARY

In an aspect, there is provided a method comprising: obtaining athin-mask transmission function of a patterning device and a M3D modelfor a lithographic process; determining a M3D mask transmission functionof the patterning device by using the thin-mask transmission functionand the M3D model; and determining an aerial image produced by thepatterning device and the lithographic process, by using the M3D masktransmission function; wherein the thin-mask transmission function is acontinuous transmission mask (CTM); wherein the M3D model at leastrepresents a portion of M3D effect attributable to multiple edges ofstructures on the patterning device.

According to an embodiment, the M3D model further represents a portionof M3D effect attributable to an edge where two sidewalls of a structureon the patterning device meet, or an edge where a sidewall of astructure on the patterning device and an area beyond a perimeter of thestructure meet.

According to an embodiment, the M3D model further represents a portionof M3D effect attributable to areas along perimeters of structures onthe patterning device.

According to an embodiment, the M3D model further represents a portionof M3D effect attributable to areas with variations of the thin-masktransmission function below a first threshold, or a portion of M3Deffect attributable to areas with variations of the thin-masktransmission function above a second threshold.

According to an embodiment, the M3D model further represents a portionof M3D effect attributable to areas of the structures away fromperimeters of structures on the patterning device.

According to an embodiment, the method further comprises determining aresist image using the aerial image.

According to an embodiment, determining the resist image comprises usinga model of a resist used in the lithographic process.

According to an embodiment, the method further comprises determining thethin-mask transmission function from structures on the patterningdevice.

According to an embodiment, the method further comprises determining thestructures from a design layout.

According to an embodiment, determining the aerial image comprises usinga model of projection optics used in the lithographic process.

According to an embodiment, determining the aerial image comprisesdetermining an electromagnetic field of a radiation after the radiationinteracts with the patterning device by using the M3D mask transmissionfunction and an electromagnetic field of the radiation before theradiation interacts with the patterning device.

According to an embodiment, the M3D mask transmission function comprisesat least a first term and a second term that respectively characterizeinteractions of a radiation with a first area and a second area of thepatterning device.

According to an embodiment, the M3D model comprises a plurality ofkernel functions and determining the M3D mask transmission functioncomprises performing an integral transform of the thin-mask transmissionfunction using the kernel functions.

According to an embodiment, the M3D model comprises a first kernelfunction and a second kernel function, and the first kernel function islinear and the second kernel function is multi-linear.

According to an embodiment, the second kernel function is bilinear.

According to an embodiment, the second kernel function is a quad-linearkernel function.

According to an embodiment, the second kernel function represents theportion of M3D effect attributable to multiple edges of structures onthe patterning device.

According to an embodiment, the quad-linear kernel function represents aportion of M3D effect attributable to an edge where two sidewalls of astructure on the patterning device meet, or edges where a sidewall of astructure on the patterning device and an area beyond the perimeter ofthe structure meet.

According to an embodiment, the second kernel function represents aportion of M3D effect attributable to areas along perimeters ofstructures on the patterning device.

According to an embodiment, the first kernel function represents theportion of M3D effect attributable to areas with variations of thethin-mask transmission function below a first threshold, and the secondkernel function represents the portion of M3D effect attributable toareas with variations of the thin-mask transmission function above asecond threshold.

According to an embodiment, the first kernel function represents theportion of M3D effect attributable to areas of the structures away fromperimeters of structures on the patterning device.

In an aspect, there is provided a computer program product comprising anon-transitory computer readable medium having instructions recordedthereon, the instructions when executed by a computer implementing anyof the methods above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of various subsystems of a lithographysystem.

FIG. 2 shows a flowchart of a method for simulation of an aerial imageor resist image where M3D is taken into account, according to anembodiment.

FIG. 3 schematically shows a flow chart for using the mask transmissionfunction.

FIG. 4 schematically shows a special case of the flow chart of FIG. 3 .

FIG. 5 schematically shows a pattern on the patterning device, as anexample to show the areas of small variations of the transmissionfunction and the areas of large variations of the transmission function.

FIG. 6A schematically shows a flow chart where M3D models may be derivedfor a number of lithographic processes and stored in a database forfuture use.

FIG. 6B schematically shows a flow chart where a M3D model may beretrieved from a database based on the lithographic process.

FIG. 7 is a block diagram of an example computer system.

FIG. 8 is a schematic diagram of a lithographic projection apparatus.

FIG. 9 is a schematic diagram of another lithographic projectionapparatus.

FIG. 10 is a more detailed view of the apparatus in FIG. 9 .

FIG. 11 is a more detailed view of the source collector module SO of theapparatus of FIG. 9 and FIG. 10 .

DETAILED DESCRIPTION

Although specific reference may be made in this text to the manufactureof ICs, it should be explicitly understood that the description hereinhas many other possible applications. For example, it may be employed inthe manufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered asinterchangeable with the more general terms “mask”, “substrate” and“target portion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange of about 5-100 nm).

The patterning device can comprise, or can form, one or more designlayouts. The design layout can be generated utilizing CAD(computer-aided design) programs, this process often being referred toas EDA (electronic design automation). Most CAD programs follow a set ofpredetermined design rules in order to create functional designlayouts/patterning devices. These rules are set by processing and designlimitations. For example, design rules define the space tolerancebetween circuit devices (such as gates, capacitors, etc.) orinterconnect lines, so as to ensure that the circuit devices or lines donot interact with one another in an undesirable way. One or more of thedesign rule limitations may be referred to as “critical dimensions”(CD). A critical dimension of a circuit can be defined as the smallestwidth of a line or hole or the smallest space between two lines or twoholes. Thus, the CD determines the overall size and density of thedesigned circuit. Of course, one of the goals in integrated circuitfabrication is to faithfully reproduce the original circuit design onthe substrate (via the patterning device).

The term “mask” or “patterning device” as employed in this text may bebroadly interpreted as referring to a generic patterning device that canbe used to endow an incoming radiation beam with a patternedcross-section, corresponding to a pattern that is to be created in atarget portion of the substrate; the term “light valve” can also be usedin this context. Besides the classic mask (transmissive or reflective;binary, phase-shifting, hybrid, etc.), examples of other such patterningdevices include:

-   -   a programmable mirror array. An example of such a device is a        matrix-addressable surface having a viscoelastic control layer        and a reflective surface. The basic principle behind such an        apparatus is that (for example) addressed areas of the        reflective surface reflect incident radiation as diffracted        radiation, whereas unaddressed areas reflect incident radiation        as undiffracted radiation. Using an appropriate filter, the said        undiffracted radiation can be filtered out of the reflected        beam, leaving only the diffracted radiation behind; in this        manner, the beam becomes patterned according to the addressing        pattern of the matrix-addressable surface. The required matrix        addressing can be performed using suitable electronic means.        More information on such mirror arrays can be gleaned, for        example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are        incorporated herein by reference.    -   a programmable LCD array. An example of such a construction is        given in U.S. Pat. No. 5,229,872, which is incorporated herein        by reference.

As a brief introduction, FIG. 1 illustrates an exemplary lithographicprojection apparatus 10A. Major components are a radiation source 12A,which may be a deep-ultraviolet excimer laser source or other type ofsource including an extreme ultra violet (EUV) source (as discussedabove, the lithographic projection apparatus itself need not have theradiation source), illumination optics which define the partialcoherence (denoted as sigma) and which may include optics 14A, 16Aa and16Ab that shape radiation from the source 12A; a patterning device 14A;and transmission optics 16Ac that project an image of the patterningdevice pattern onto a substrate plane 22A. An adjustable filter oraperture 20A at the pupil plane of the projection optics may restrictthe range of beam angles that impinge on the substrate plane 22A, wherethe largest possible angle defines the numerical aperture of theprojection optics NA=n sin(Θ_(max)), n is the index of refraction of themedia between the last element of projection optics and the substrate,and Θ_(max) is the largest angle of the beam exiting from the projectionoptics that can still impinge on the substrate plane 22A.

In a lithographic projection apparatus, a source provides illumination(i.e. radiation) to a patterning device and projection optics direct andshape the illumination, via the patterning device, onto a substrate. Theprojection optics may include at least some of the components 14A, 16Aa,16Ab and 16Ac. An aerial image (AI) is the radiation intensitydistribution at substrate level. A resist layer on the substrate isexposed and the aerial image is transferred to the resist layer as alatent “resist image” (RI) therein. The resist image (RI) can be definedas a spatial distribution of solubility of the resist in the resistlayer. A resist model can be used to calculate the resist image from theaerial image, an example of which can be found in U.S. PatentApplication Publication No. US 2009-0157360, the disclosure of which ishereby incorporated by reference in its entirety. The resist model isrelated only to properties of the resist layer (e.g., effects ofchemical processes which occur during exposure, PEB and development).Optical properties of the lithographic projection apparatus (e.g.,properties of the source, the patterning device and the projectionoptics) dictate the aerial image. Since the patterning device used inthe lithographic projection apparatus can be changed, it may bedesirable to separate the optical properties of the patterning devicefrom the optical properties of the rest of the lithographic projectionapparatus including at least the source and the projection optics.

One aspect of understanding a lithographic process is understanding theinteraction of the radiation and the patterning device. Theelectromagnetic field of the radiation after the radiation passes thepatterning device may be determined from the electromagnetic field ofthe radiation before the radiation reaches the patterning device and afunction that characterizes the interaction. This function may bereferred to as the mask transmission function.

The mask transmission function may have a variety of different forms.One form is binary. A binary mask transmission function has either oftwo values (e.g., zero and a positive constant) at any given location onthe patterning device. A mask transmission function in the binary formmay be referred to as a binary mask. Another form is continuous. Namely,the modulus of the transmittance of the patterning device is acontinuous function of the location on the patterning device. The phaseof the transmittance may also be a continuous function of the locationon the patterning device. A mask transmission function in the continuousform may be referred to as a continuous transmission mask (CTM).

The thin-mask approximation, also called the Kirchhoff boundarycondition, is widely used to simplify the determination of theinteraction of the radiation and the patterning device. The thin-maskapproximation assumes that the thickness of the structures on thepatterning device is very small compared with the wavelength and thatthe widths of the structures on the mask are very large compared withthe wavelength. Therefore, the thin-mask approximation assumes theelectromagnetic field after the patterning device is the multiplicationof the incident electromagnetic field with the mask transmissionfunction. However, as lithographic processes use radiation of shorterand shorter wavelengths, and the structures on the patterning device(“mask 3D” or “M3D”) become smaller and smaller, the assumption of thethin-mask approximation can break down. For example, the interaction ofradiation with the structures (e.g., edges between the top surface and asidewall) because of their finite thicknesses (“mask 3D effect” or “M3Deffect”) may become significant. Encompassing this scattering in themask transmission function may make the mask transmission functionbetter at capturing the interaction of the radiation with the patterningdevice. A mask transmission function under the thin-mask approximationmay be referred to as a thin-mask transmission function. A masktransmission function encompassing a M3D effect may be referred to as aM3D mask transmission function.

The M3D mask transmission function may be obtained by rigoroussimulation such as a Finite-Discrete-Time-Domain (FDTD) algorithm or aRigorous-Coupled Waveguide Analysis (RCWA) algorithm. However, rigoroussimulation can be computationally expensive. Another approach is torigorously simulate the M3D effect of certain portions of the structuresthat tend to have a large M3D effect, and add the M3D effect of theseportions to a thin-mask transmission function. Although this approach isless computationally expensive, it still involves rigorous simulation.

In this disclosure, a method is disclosed that determines the M3D effectof structures on a patterning device from the thin-mask transmissionfunction of the patterning device.

FIG. 2 is a flowchart of a method for determining an aerial image orresist image where M3D is taken into account, according to anembodiment. In procedure 2005, a thin-mask transmission function 2003 ofa patterning device and a M3D model 2004 are used to determine a M3Dmask transmission function 2006 of the patterning device. A M3D model isa model that models the M3D from a thin-mask transmission function. Thethin-mask transmission function 2003 may be determined from structures2002 on the patterning device. The structures 2002 may be determinedfrom a design layout 2001. In procedure 2008, the M3D mask transmissionfunction 2006 and a projection optics model 2007 are used to determine(e.g., simulate) an aerial image 2009. The aerial image 2009 and aresist model 2010 may be used in optional procedure 2011 to determine(e.g., simulate) a resist image 2012.

The mask transmission function (e.g., thin-mask or M3D) of a patterningdevice is a function that links the electromagnetic field of theradiation before it interacts with the patterning device and theelectromagnetic field of the radiation after it interacts with thepatterning device. FIG. 3 schematically shows a flow chart for using themask transmission function. The electromagnetic field 3001 of theradiation before it interacts with the patterning device and the masktransmission function 3002 are used in procedure 3003 to determine theelectromagnetic field 3004 of the radiation after it interacts with thepatterning device. The mask transmission function 3002 may be athin-mask transmission function. The mask transmission function 3002 maybe a M3D mask transmission function. In a generic mathematical form, therelationship between the electromagnetic field 3001 and theelectromagnetic field 3004 may be expressed as E_(a)(r)=T(E_(b)(r)),wherein E_(a)(r) is the electric component of the electromagnetic field3004, E_(b)(r) is the electric component of the electromagnetic field3001, and T is the mask transmission function.

FIG. 4 schematically shows the flow of FIG. 3 where the masktransmission function 3002 is a M3D mask transmission function and is asum of at least two terms 3002A and 3002B, where the terms 3002A and3002B respectively characterize the interactions of the radiation withdifferent areas of the patterning device. The electromagnetic field 3001and the term 3002A are used in a sub-procedure 3003A of the procedure3003 to determine a portion 3004A of the electromagnetic field 3004,where the portion 3004A is a result of the interaction of the radiation(as represented by the electromagnetic field 3001) with a first area ofthe patterning device. The electromagnetic field 3001 and the term 3002Bare used in a sub-procedure 3003B of the procedure 3003 to determine aportion 3004B of the electromagnetic field 3004, where the portion 3004Bis a result of the interaction of the radiation (as represented by theelectromagnetic field 3001) with a second area of the patterning device.The electromagnetic field 3004 may be approximated by the sum of theportions 3004A and 3004B.

The M3D model 2004 in FIG. 2 may include one or more kernel functions.The procedure 2005 in FIG. 2 may include performing an integraltransform of the thin-mask transmission function 2003 using the one ormore kernel functions.

According to an embodiment, the kernel functions may include a linearkernel function and a multi-linear (e.g., bilinear) kernel function. Thelinear kernel function may represent a portion of the M3D effectattributable to areas of relatively small variations of the thin-masktransmission function. For example, when the thin-mask transmissionfunction is a binary transmission function, the areas of smallvariations may include flat areas (i.e., away from areas with thicknesschanges) of the structures on the patterning device. When the thin-masktransmission function is a CTM, the areas of small variations mayinclude areas where derivatives of the phase and modulus with respect topositions are below a threshold. The multi-linear kernel function mayrepresent portions of the M3D effect attributable to areas of relativelylarge variations of the thin-mask transmission function. For example,when the thin-mask transmission function is a binary transmissionfunction, the areas of large variations may include areas near edges andcorners (i.e., near changes of the thickness) of the structures on thepatterning device. When the thin-mask transmission function is a CTM,the areas of large variations may include areas where derivatives of thephase and modulus with respect to positions are above a threshold. Themulti-linear kernel function may represent portions of the M3D effectattributable to areas that contain two or more large variations of thetransmission function, for example, areas including two edges close toeach other.

FIG. 5 schematically shows a pattern 5000 on the patterning device, asan example to show areas of relatively small variations of thetransmission function and areas of relatively large variations of thetransmission function. The pattern 5000 has a finite thickness. Thepattern 5000 is defined by sidewalls along its perimeter, where, withinthe perimeter, the thickness is a finite positive constant and thethickness is zero beyond the perimeter. Areas of relatively smallvariations would include the area 5010 in the interior of the pattern5000, away from the perimeter. The portion of M3D effect attributable tothe area 5010 may be represented by a linear kernel function. Areas ofrelatively large variations would include the areas 5001-5006 alongedges of the perimeter and away from corners, and the areas 5021-5026near the corners. The portion of M3D effect attributable of these areas5001-5006 and 5021-5026 may be represented by the multi-linear kernelfunction.

In an example, the M3D mask transmission function may be derived fromthe thin-mask transmission function using the following formula:∫m(r₁)m*(r₂)T(r−r₁,r−r₂)dr₁dr₂+∫m(r₁)R(r−r₁)dr₁wherein m(r) is the thin-mask transmission function, T is a bilinearkernel function as an example of the multi-linear kernel function, and Ris a linear kernel function.

In another example, the M3D mask transmission function may be derivedfrom the thin-mask transmission function using the following formula:{∫[∫m(r₁)F(r₂−r₁)dr₁]²G(r−r₂)dr₂}+∫m(r₁)R(r−r₁)dr₁Namely, the multi-linear kernel may be approximated by a kernel functionF that detects edges and a kernel function G that derives the M3D effectof the edges. In this approximation, the M3D effect attributable toareas that contain two or more large variations of the transmissionfunction (e.g., areas including two edges close to each other) areignored.

The multi-linear kernel function may include kernel functions of higherorders than bilinear kernel functions. For example, the kernel functionsmay include a quad-linear kernel function, which may represent portionsof the M3D effect attributable to edges where two sidewalls meet, oredges where a sidewall and an area beyond the perimeter of a structuremeet.

The M3D model (e.g., as represented by the kernel functions) may varywith the lithographic process (e.g., as represented by one or morecharacteristics of the radiation and one or more characteristics of thepatterning device). The M3D model may be derived for a particularlithographic process. The M3D model (e.g., as represented by the kernelfunctions) may be obtained by simulation.

FIG. 6A schematically shows a flow chart where M3D models may be derivedfor a number of lithographic processes and stored in a database forfuture use. One or more characteristics of a lithographic process 6001are used to derive a M3D model 6003 for the lithographic process 6001 inprocedure 6002. The M3D model 6003 may be obtained by simulation. TheM3D model 6003 is stored in a database 6004.

FIG. 6B schematically shows a flow chart where a M3D model may beretrieved from a database based on the lithographic process. Inprocedure 6005, one or more characteristics of a lithographic process6001 are used to query the database 6004 and retrieve a M3D model 6003for the lithographic process 6001.

FIG. 7 is a block diagram that illustrates a computer system 100 whichcan assist in implementing the methods, flows or the apparatus disclosedherein. Computer system 100 includes a bus 102 or other communicationmechanism for communicating information, and a processor 104 (ormultiple processors 104 and 105) coupled with bus 102 for processinginformation. Computer system 100 also includes a main memory 106, suchas a random access memory (RAM) or other dynamic storage device, coupledto bus 102 for storing information and instructions to be executed byprocessor 104. Main memory 106 also may be used for storing temporaryvariables or other intermediate information during execution ofinstructions to be executed by processor 104. Computer system 100further includes a read only memory (ROM) 108 or other static storagedevice coupled to bus 102 for storing static information andinstructions for processor 104. A storage device 110, such as a magneticdisk or optical disk, is provided and coupled to bus 102 for storinginformation and instructions.

Computer system 100 may be coupled via bus 102 to a display 112, such asa cathode ray tube (CRT) or flat panel or touch panel display fordisplaying information to a computer user. An input device 114,including alphanumeric and other keys, is coupled to bus 102 forcommunicating information and command selections to processor 104.Another type of user input device is cursor control 116, such as amouse, a trackball, or cursor direction keys for communicating directioninformation and command selections to processor 104 and for controllingcursor movement on display 112. This input device typically has twodegrees of freedom in two axes, a first axis (e.g., x) and a second axis(e.g., y), that allows the device to specify positions in a plane. Atouch panel (screen) display may also be used as an input device.

According to one embodiment, portions of a process described herein maybe performed by computer system 100 in response to processor 104executing one or more sequences of one or more instructions contained inmain memory 106. Such instructions may be read into main memory 106 fromanother computer-readable medium, such as storage device 110. Executionof the sequences of instructions contained in main memory 106 causesprocessor 104 to perform one or more of the process steps describedherein. One or more processors in a multi-processing arrangement mayalso be employed to execute the sequences of instructions contained inmain memory 106. In an alternative embodiment, hard-wired circuitry maybe used in place of or in combination with software instructions. Thus,the description herein is not limited to any specific combination ofhardware circuitry and software.

The term “computer-readable medium” as used herein refers to any mediumthat participates in providing instructions to processor 104 forexecution. Such a medium may take many forms, including but not limitedto, non-volatile media, volatile media, and transmission media.Non-volatile media include, for example, optical or magnetic disks, suchas storage device 110. Volatile media include dynamic memory, such asmain memory 106. Transmission media include coaxial cables, copper wireand fiber optics, including the wires that comprise bus 102.Transmission media can also take the form of acoustic or light waves,such as those generated during radio frequency (RF) and infrared (IR)data communications. Common forms of computer-readable media include,for example, a floppy disk, a flexible disk, hard disk, magnetic tape,any other magnetic medium, a CD-ROM, DVD, any other optical medium,punch cards, paper tape, any other physical medium with patterns ofholes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip orcartridge, a carrier wave as described hereinafter, or any other mediumfrom which a computer can read.

Various forms of computer readable media may be involved in carrying oneor more sequences of one or more instructions to processor 104 forexecution. For example, the instructions may initially be borne on amagnetic disk of a remote computer. The remote computer can load theinstructions into its dynamic memory and send the instructions over atelephone line using a modem. A modem local to computer system 100 canreceive the data on the telephone line and use an infrared transmitterto convert the data to an infrared signal. An infrared detector coupledto bus 102 can receive the data carried in the infrared signal and placethe data on bus 102. Bus 102 carries the data to main memory 106, fromwhich processor 104 retrieves and executes the instructions. Theinstructions received by main memory 106 may optionally be stored onstorage device 110 either before or after execution by processor 104.

Computer system 100 may also include a communication interface 118coupled to bus 102. Communication interface 118 provides a two-way datacommunication coupling to a network link 120 that is connected to alocal network 122. For example, communication interface 118 may be anintegrated services digital network (ISDN) card or a modem to provide adata communication connection to a corresponding type of telephone line.As another example, communication interface 118 may be a local areanetwork (LAN) card to provide a data communication connection to acompatible LAN. Wireless links may also be implemented. In any suchimplementation, communication interface 118 sends and receiveselectrical, electromagnetic or optical signals that carry digital datastreams representing various types of information.

Network link 120 typically provides data communication through one ormore networks to other data devices. For example, network link 120 mayprovide a connection through local network 122 to a host computer 124 orto data equipment operated by an Internet Service Provider (ISP) 126.ISP 126 in turn provides data communication services through theworldwide packet data communication network, now commonly referred to asthe “Internet” 128. Local network 122 and Internet 128 both useelectrical, electromagnetic or optical signals that carry digital datastreams. The signals through the various networks and the signals onnetwork link 120 and through communication interface 118, which carrythe digital data to and from computer system 100, are exemplary forms ofcarrier waves transporting the information.

Computer system 100 can send messages and receive data, includingprogram code, through the network(s), network link 120, andcommunication interface 118. In the Internet example, a server 130 mighttransmit a requested code for an application program through Internet128, ISP 126, local network 122 and communication interface 118. Onesuch downloaded application may provide for one or more process stepsdescribed herein, for example. The received code may be executed byprocessor 104 as it is received, and/or stored in storage device 110, orother non-volatile storage for later execution. In this manner, computersystem 100 may obtain application code in the form of a carrier wave.

FIG. 8 schematically depicts an exemplary lithographic projectionapparatus whose illumination could be optimized utilizing the methodsdescribed herein. The apparatus comprises:

-   -   an illumination system IL, to condition a beam B of radiation.        In this particular case, the illumination system also comprises        a radiation source SO;    -   a first object table (e.g., patterning device table) MT provided        with a patterning device holder to hold a patterning device MA        (e.g., a reticle), and connected to a first positioner to        accurately position the patterning device with respect to item        PS;    -   a second object table (substrate table) WT provided with a        substrate holder to hold a substrate W (e.g., a resist-coated        silicon wafer), and connected to a second positioner to        accurately position the substrate with respect to item PS;    -   a projection system (“lens”) PS (e.g., a refractive, catoptric        or catadioptric optical system) to image an irradiated portion        of the patterning device MA onto a target portion C (e.g.,        comprising one or more dies) of the substrate W.

As depicted herein, the apparatus is of a transmissive type (i.e., has atransmissive patterning device). However, in general, it may also be ofa reflective type, for example (with a reflective patterning device).The apparatus may employ a different kind of patterning device toclassic mask; examples include a programmable mirror array or LCDmatrix.

The source SO (e.g., a mercury lamp or excimer laser, LPP (laserproduced plasma) EUV source) produces a beam of radiation. This beam isfed into an illumination system (illuminator) IL, either directly orafter having traversed conditioning means, such as a beam expander Ex,for example. The illuminator IL may comprise adjusting means AD forsetting the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in thebeam. In addition, it will generally comprise various other components,such as an integrator IN and a condenser CO. In this way, the beam Bimpinging on the patterning device MA has a desired uniformity andintensity distribution in its cross-section.

It should be noted with regard to FIG. 8 that the source SO may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source SO is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam that it produces being led into the apparatus (e.g., withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source SO is an excimer laser (e.g., based on KrF, ArFor F₂ lasing).

The beam PB subsequently intercepts the patterning device MA, which isheld on a patterning device table MT. Having traversed the patterningdevice MA, the beam B passes through the lens PL, which focuses the beamB onto a target portion C of the substrate W. With the aid of the secondpositioning means (and interferometric measuring means IF), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the beam PB. Similarly, thefirst positioning means can be used to accurately position thepatterning device MA with respect to the path of the beam B, e.g., aftermechanical retrieval of the patterning device MA from a patterningdevice library, or during a scan. In general, movement of the objecttables MT, WT will be realized with the aid of a long-stroke module(coarse positioning) and a short-stroke module (fine positioning), whichare not explicitly depicted in FIG. 8 . However, in the case of astepper (as opposed to a step-and-scan tool) the patterning device tableMT may just be connected to a short stroke actuator, or may be fixed.

The depicted tool can be used in two different modes:

-   -   In step mode, the patterning device table MT is kept essentially        stationary, and an entire patterning device image is projected        in one go (i.e., a single “flash”) onto a target portion C. The        substrate table WT is then shifted in the x and/or y directions        so that a different target portion C can be irradiated by the        beam PB;    -   In scan mode, essentially the same scenario applies, except that        a given target portion C is not exposed in a single “flash”.        Instead, the patterning device table MT is movable in a given        direction (the so-called “scan direction”, e.g., the y        direction) with a speed v, so that the projection beam B is        caused to scan over a patterning device image; concurrently, the        substrate table WT is simultaneously moved in the same or        opposite direction at a speed V=Mv, in which M is the        magnification of the lens PL (typically, M=¼ or ⅕). In this        manner, a relatively large target portion C can be exposed,        without having to compromise on resolution.

FIG. 9 schematically depicts another exemplary lithographic projectionapparatus 1000 whose illumination could be optimized utilizing themethods described herein.

The lithographic projection apparatus 1000 comprises:

-   -   a source collector module SO    -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. EUV radiation).    -   a support structure (e.g. a patterning device table) MT        constructed to support a patterning device (e.g. a mask or a        reticle) MA and connected to a first positioner PM configured to        accurately position the patterning device;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate; and    -   a projection system (e.g. a reflective projection system) PS        configured to project a pattern imparted to the radiation beam B        by patterning device MA onto a target portion C (e.g. comprising        one or more dies) of the substrate W.

As here depicted, the apparatus 1000 is of a reflective type (e.g.employing a reflective patterning device). It is to be noted thatbecause most materials are absorptive within the EUV wavelength range,the patterning device may have multilayer reflectors comprising, forexample, a multi-stack of Molybdenum and Silicon. In one example, themulti-stack reflector has a 40 layer pairs of Molybdenum and Siliconwhere the thickness of each layer is a quarter wavelength. Even smallerwavelengths may be produced with X-ray lithography. Since most materialis absorptive at EUV and x-ray wavelengths, a thin piece of patternedabsorbing material on the patterning device topography (e.g., a TaNabsorber on top of the multi-layer reflector) defines where featureswould print (positive resist) or not print (negative resist).

Referring to FIG. 9 , the illuminator IL receives an extreme ultraviolet radiation beam from the source collector module SO. Methods toproduce EUV radiation include, but are not necessarily limited to,converting a material into a plasma state that has at least one element,e.g., xenon, lithium or tin, with one or more emission lines in the EUVrange. In one such method, often termed laser produced plasma (“LPP”)the plasma can be produced by irradiating a fuel, such as a droplet,stream or cluster of material having the line-emitting element, with alaser beam. The source collector module SO may be part of an EUVradiation system including a laser, not shown in FIG. 9 , for providingthe laser beam exciting the fuel. The resulting plasma emits outputradiation, e.g., EUV radiation, which is collected using a radiationcollector, disposed in the source collector module. The laser and thesource collector module may be separate entities, for example when a CO2laser is used to provide the laser beam for fuel excitation.

In such cases, the laser is not considered to form part of thelithographic apparatus and the radiation beam is passed from the laserto the source collector module with the aid of a beam delivery systemcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thesource collector module, for example when the source is a dischargeproduced plasma EUV generator, often termed as a DPP source.

The illuminator IL may comprise an adjuster for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as facetted field and pupilmirror devices. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., patterning devicetable) MT, and is patterned by the patterning device. After beingreflected from the patterning device (e.g. mask) MA, the radiation beamB passes through the projection system PS, which focuses the beam onto atarget portion C of the substrate W. With the aid of the secondpositioner PW and position sensor PS2 (e.g. an interferometric device,linear encoder or capacitive sensor), the substrate table WT can bemoved accurately, e.g. so as to position different target portions C inthe path of the radiation beam B. Similarly, the first positioner PM andanother position sensor PS1 can be used to accurately position thepatterning device (e.g. mask) MA with respect to the path of theradiation beam B. Patterning device (e.g. mask) MA and substrate W maybe aligned using patterning device alignment marks M1, M2 and substratealignment marks P1, P2.

The depicted apparatus 1000 could be used in at least one of thefollowing modes:

1. In step mode, the support structure (e.g. patterning device table) MTand the substrate table WT are kept essentially stationary, while anentire pattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e. a single static exposure). The substratetable WT is then shifted in the X and/or Y direction so that a differenttarget portion C can be exposed.

2. In scan mode, the support structure (e.g. patterning device table) MTand the substrate table WT are scanned synchronously while a patternimparted to the radiation beam is projected onto a target portion C(i.e. a single dynamic exposure). The velocity and direction of thesubstrate table WT relative to the support structure (e.g. patterningdevice table) MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS.

3. In another mode, the support structure (e.g. patterning device table)MT is kept essentially stationary holding a programmable patterningdevice, and the substrate table WT is moved or scanned while a patternimparted to the radiation beam is projected onto a target portion C. Inthis mode, generally a pulsed radiation source is employed and theprogrammable patterning device is updated as required after eachmovement of the substrate table WT or in between successive radiationpulses during a scan. This mode of operation can be readily applied tomaskless lithography that utilizes programmable patterning device, suchas a programmable mirror array of a type as referred to above.

FIG. 10 shows the apparatus 1000 in more detail, including the sourcecollector module SO, the illumination system IL, and the projectionsystem PS. The source collector module SO is constructed and arrangedsuch that a vacuum environment can be maintained in an enclosingstructure 220 of the source collector module SO. An EUV radiationemitting plasma 210 may be formed by a discharge produced plasma source.EUV radiation may be produced by a gas or vapor, for example Xe gas, Livapor or Sn vapor in which the very hot plasma 210 is created to emitradiation in the EUV range of the electromagnetic spectrum. The very hotplasma 210 is created by, for example, an electrical discharge causingat least partially ionized plasma. Partial pressures of, for example, 10Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may berequired for efficient generation of the radiation. In an embodiment, aplasma of excited tin (Sn) is provided to produce EUV radiation.

The radiation emitted by the hot plasma 210 is passed from a sourcechamber 211 into a collector chamber 212 via an optional gas barrier orcontaminant trap 230 (in some cases also referred to as contaminantbarrier or foil trap) which is positioned in or behind an opening insource chamber 211. The contaminant trap 230 may include a channelstructure. Contamination trap 230 may also include a gas barrier or acombination of a gas barrier and a channel structure. The contaminanttrap or contaminant barrier 230 further indicated herein at leastincludes a channel structure, as known in the art.

The collector chamber 211 may include a radiation collector CO which maybe a so-called grazing incidence collector. Radiation collector CO hasan upstream radiation collector side 251 and a downstream radiationcollector side 252. Radiation that traverses collector CO can bereflected off a grating spectral filter 240 to be focused in a virtualsource point IF along the optical axis indicated by the dot-dashed line‘O’. The virtual source point IF is commonly referred to as theintermediate focus, and the source collector module is arranged suchthat the intermediate focus IF is located at or near an opening 221 inthe enclosing structure 220. The virtual source point IF is an image ofthe radiation emitting plasma 210.

Subsequently the radiation traverses the illumination system IL, whichmay include a facetted field mirror device 22 and a facetted pupilmirror device 24 arranged to provide a desired angular distribution ofthe radiation beam 21, at the patterning device MA, as well as a desireduniformity of radiation intensity at the patterning device MA. Uponreflection of the beam of radiation 21 at the patterning device MA, heldby the support structure MT, a patterned beam 26 is formed and thepatterned beam 26 is imaged by the projection system PS via reflectiveelements 28, 30 onto a substrate W held by the substrate table WT.

More elements than shown may generally be present in illumination opticsunit IL and projection system PS. The grating spectral filter 240 mayoptionally be present, depending upon the type of lithographicapparatus. Further, there may be more mirrors present than those shownin the figures, for example there may be 1-6 additional reflectiveelements present in the projection system PS than shown in FIG. 10 .

Collector optic CO, as illustrated in FIG. 10 , is depicted as a nestedcollector with grazing incidence reflectors 253, 254 and 255, just as anexample of a collector (or collector mirror). The grazing incidencereflectors 253, 254 and 255 are disposed axially symmetric around theoptical axis O and a collector optic CO of this type may be used incombination with a discharge produced plasma source, often called a DPPsource.

Alternatively, the source collector module SO may be part of an LPPradiation system as shown in FIG. 11 . A laser LA is arranged to depositlaser energy into a fuel, such as xenon (Xe), tin (Sn) or lithium (Li),creating the highly ionized plasma 210 with electron temperatures ofseveral 10's of eV. The energetic radiation generated duringde-excitation and recombination of these ions is emitted from theplasma, collected by a near normal incidence collector optic CO andfocused onto the opening 221 in the enclosing structure 220.

U.S. Patent Application Publication No. US 2013-0179847 is herebyincorporated by reference in its entirety.

The embodiments may further be described using the following clauses:

1. A method comprising:

obtaining a thin-mask transmission function of a patterning device and aM3D model for a lithographic process, wherein the thin-mask transmissionfunction is a continuous transmission mask and the M3D model at leastrepresents a portion of M3D effect attributable to multiple edges of astructure on the patterning device;

determining a M3D mask transmission function of the patterning device byusing the thin-mask transmission function and the M3D model; and

determining an aerial image produced by the patterning device and thelithographic process, by using the M3D mask transmission function.

2. The method of clause 1, wherein the M3D model further represents aportion of M3D effect attributable to an edge where two sidewalls of astructure on the patterning device meet, or attributable to an edgewhere a sidewall of a structure on the patterning device and an areabeyond a perimeter of the structure meet.3. The method of clause 1 or clause 2, wherein the M3D model furtherrepresents a portion of M3D effect attributable to an area along aperimeter of a structure on the patterning device.4. The method of any of clauses 1-3, wherein the M3D model furtherrepresents a portion of M3D effect attributable to an areas with avariation of the thin-mask transmission function below a firstthreshold, or a portion of M3D effect attributable to an area with avariation of the thin-mask transmission function above a secondthreshold.5. The method of any of clauses 1-4, wherein the M3D model furtherrepresents a portion of M3D effect attributable to an area of astructure away from a perimeter of the structure on the patterningdevice.6. The method of any of clauses 1-5, further comprising determining aresist image using the aerial image.7. The method of clause 6, wherein determining the resist imagecomprises using a model of a resist used in the lithographic process.8. The method of any of clauses 1-7, further comprising determining thethin-mask transmission function from structures on the patterningdevice.9. The method of clause 8, further comprising determining the structuresfrom a design layout.10. The method of any of clauses 1-9, wherein determining the aerialimage comprises using a model of projection optics used in thelithographic process.11. The method of any of clauses 1-10, wherein determining the aerialimage comprises determining an electromagnetic field of radiation afterthe radiation interacts with the patterning device by using the M3D masktransmission function and an electromagnetic field of the radiationbefore the radiation interacts with the patterning device.12. The method of any of clauses 1-11, wherein the M3D mask transmissionfunction comprises at least a first term and a second term thatrespectively characterize interactions of a radiation with a first areaand a second area of the patterning device.13. The method of any of clauses 1-12, wherein the M3D model comprises aplurality of kernel functions and determining the M3D mask transmissionfunction comprises performing an integral transform of the thin-masktransmission function using the kernel functions.14. The method of any of clauses 1-13, wherein the M3D model comprises afirst kernel function and a second kernel function, wherein the firstkernel function is linear and the second kernel function ismulti-linear.15. The method of clause 14, wherein the second kernel function isbilinear.16. The method of clause 14, wherein the second kernel function is aquad-linear kernel function.17. The method of clause 16, wherein the quad-linear kernel functionrepresents a portion of M3D effect attributable to an edge where twosidewalls of a structure on the patterning device meet, or attributableto an edge where a sidewall of a structure on the patterning device andan area beyond the perimeter of the structure meet.18. The method of any of clauses 14-17, wherein the second kernelfunction represents the portion of M3D effect attributable to multipleedges of structures on the patterning device.19. The method of any of clauses 14-18, wherein the second kernelfunction represents a portion of M3D effect attributable to an areaalong a perimeter of a structure on the patterning device.20. The method of any of clauses 14-19, wherein the first kernelfunction represents the portion of M3D effect attributable to an areawith a variation of the thin-mask transmission function below a firstthreshold, and the second kernel function represents the portion of M3Deffect attributable to an area with a variation of the thin-masktransmission function above a second threshold.21. The method of any of clauses 14-20, wherein the first kernelfunction represents the portion of M3D effect attributable to an area ofa structure away from a perimeter of the structure on the patterningdevice.22. A computer program product comprising a non-transitory computerreadable medium having instructions recorded thereon, the instructionswhen executed by a computer implementing a method of any of clauses1-21.

The concepts disclosed herein may simulate or mathematically model anygeneric imaging system for imaging sub wavelength features, and may beespecially useful with emerging imaging technologies capable ofproducing increasingly shorter wavelengths. Emerging technologiesalready in use include EUV (extreme ultra violet), DUV lithography thatis capable of producing a 193 nm wavelength with the use of an ArFlaser, and even a 157 nm wavelength with the use of a Fluorine laser.Moreover, EUV lithography is capable of producing wavelengths within arange of 20-5 nm by using a synchrotron or by hitting a material (eithersolid or a plasma) with high energy electrons in order to producephotons within this range.

While the concepts disclosed herein may be used for imaging on asubstrate such as a silicon wafer, it shall be understood that thedisclosed concepts may be used with any type of lithographic imagingsystems, e.g., those used for imaging on substrates other than siliconwafers.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made as described without departing from the scope of the claimsset out below.

What is claimed is:
 1. A method comprising: obtaining a 2D continuoustransmission representation of a pattern for a patterning device for alithographic process; determining a 3D transmission representation ofthe pattern from the 2D continuous transmission representation; anddetermining, by a hardware computer using the 3D transmissionrepresentation, a simulated image as would be produced by using thepattern and the lithographic process.
 2. The method of claim 1, whereinthe determining the 3D transmission representation comprises applying adifferent function to one or more edges of the pattern compared toportions located away from the one or more edges of the pattern.
 3. Themethod of claim 1, wherein the determining the 3D transmissionrepresentation comprises using a mask 3D model for the lithographicprocess, wherein the mask 3D model at least represents a portion of mask3D effect attributable to a structure on the patterning device.
 4. Themethod of claim 1, wherein the determining the 3D transmissionrepresentation comprises performing an integral transform of a functioncorresponding to the 2D continuous transmission representation using aplurality of kernel functions.
 5. The method of claim 1, wherein thedetermining the 3D transmission representation comprises using a firstkernel function and a second kernel function, wherein the first kernelfunction is linear and the second kernel function is multi-linear. 6.The method of claim 5, wherein the second kernel function is a bilinearor a quad-linear kernel function.
 7. The method of claim 5, wherein thesecond kernel function represents a portion of mask 3D effectattributable to multiple edges of structures on the patterning device orattributable to an area along a perimeter of a structure on thepatterning device.
 8. The method of claim 5, wherein the first kernelfunction represents a portion of mask 3D effect attributable to an areawith a variation of the 2D continuous transmission representation belowa first threshold, and the second kernel function represents a portionof mask 3D effect attributable to an area with a variation of the 2Dcontinuous transmission representation above a second threshold.
 9. Themethod of claim 1, wherein the 3D transmission representation is in theform of a function.
 10. The method of claim 9, wherein the 3Dtransmission function comprises at least a first term and a second termthat respectively characterize interactions of a radiation with a firstarea and a second area of the patterning device.
 11. A computer programproduct comprising a non-transitory computer readable medium havinginstructions therein, the instructions, when executed by a computersystem, configured to cause the computer system to at least: obtain a 2Dcontinuous transmission representation of a pattern for a patterningdevice for a lithographic process; determine a 3D transmissionrepresentation of the pattern from the 2D continuous transmissionrepresentation; and determine, using the 3D transmission representation,a simulated image as would be produced by using the pattern and thelithographic process.
 12. The computer program product of claim 11,wherein the instructions configured to cause the computer system todetermine the 3D transmission representation are further configured tocause the computer system to apply a different function to one or moreedges of the pattern compared to portions located away from the one ormore edges of the pattern.
 13. The computer program product of claim 11,wherein the instructions configured to cause the computer system todetermine the 3D transmission representation are further configured tocause the computer system to use a mask 3D model for the lithographicprocess, wherein the mask 3D model at least represents a portion of mask3D effect attributable to a structure on the patterning device.
 14. Thecomputer program product of claim 11, wherein the instructionsconfigured to cause the computer system to determine the 3D transmissionrepresentation are further configured to cause the computer system toperform an integral transform of a function corresponding to the 2Dtransmission representation using a plurality of kernel functions. 15.The computer program product of claim 11, wherein the instructionsconfigured to cause the computer system to determine the 3D transmissionrepresentation are further configured to cause the computer system touse a first kernel function and a second kernel function, wherein thefirst kernel function is linear and the second kernel function ismulti-linear.
 16. The computer program product of claim 15, wherein thesecond kernel function is a bilinear or a quad-linear kernel function.17. The computer program product of claim 11, wherein the 3D continuoustransmission representation is in the form of a function.
 18. Thecomputer program product of claim 17, wherein the 3D transmissionfunction comprises at least a first term and a second term thatrespectively characterize interactions of a radiation with a first areaand a second area of the patterning device.
 19. A method comprising:obtaining a 2D transmission representation of a pattern for a patterningdevice for a lithographic process; determining a 3D continuoustransmission representation of the pattern from the 2D transmissionrepresentation; and determining, by a hardware computer using the 3Dcontinuous transmission representation, a simulated image as would beproduced by using the pattern and the lithographic process.
 20. Acomputer program product comprising a non-transitory computer readablemedium having instructions therein, the instructions, when executed by acomputer system, configured to cause the computer system to at least:obtain a 2D transmission representation of a pattern for a patterningdevice for a lithographic process; determine a 3D continuoustransmission representation of the pattern from the 2D transmissionrepresentation; and determine, using the 3D continuous transmissionrepresentation, a simulated image as would be produced by using thepattern and the lithographic process.